In designing high frequency circuits, it is usually desirable to obtain "solid" ground connections which present a low impedance both at DC and at high frequencies. This is obtained in some instances by metallizing the back of the circuit board to provide a backplane which is grounded by soldering the backplane to a good ground and/or heat sink. The connection from the circuit side of the substrate to the backplane may be made by wrap around metallization. However, the use of wrap around edge metallization limits the number of substrates that can be produced from a single chip and the grounding is less than ideal if there are many connections to be made and especially if these connections extend physically a long distance from the edge of the chip. It is much more desirable to have holes in the circuit board which provide a direct connection from the circuit side to the backplane side. Such direct connections vastly reduce circuit complexity but introduce new problems. If mere plated-through holes are utilized, not only does solder flow from the backplane side to the circuit side with resultant damage to the closely spaced circuit paths, but some of the prior art techniques have left the circuit board with an uneven surface and the holes and/or the uneven surface have caused problems with subsequent metallization processes.
The present invention attempts to solve these problems by performing the plating in such a fashion that the circuit side of the substrate remains flush due to a cap which is formed at the circuit side of the plate hole and this cap in addition to providing a smooth surface prevents the flow of solder when the substrate is being soldered to a heat sink or grounding device.
It is thus an object of the present invention to provide an improved plated hole process and resulting improved product.